The standard pyrometallurgical process of lead smelting has been a combination of sintering and reduction. This conventional process comprises subjecting lead concentrate (mainly PbS) to desulfurization sintering to form sinter, i.e., sintered lumps of PbO, and reducing the sinter in a smelting furnace with a reducing agent such as coke to form crude lead and slag.
This process has various defects. A compact sintering apparatus suitable for treatment of lead concentrate is not available. Since the sulfur dioxide concentration in the exhaust gas is low, collection of said gas is difficult. As the sintering is performed at a lower temperature, hydrocarbons remain in the exhaust gas, which discolor the sulfuric acid produced from the recovered sulfur dioxide. Therefore, additional purification treatment is required for decoloring the formed sulfuric acid in order to obtain sulfuric acid having commercial value.
Rather recently flash smelting processes have been developed in the field of non-ferrous metallurgy. Typical of these are the Outokumpu process and the Kivcet process, which are now applied to lead smelting. These processes are reviewed in Journal of Metal, December, 1966, p. 1298ff and November, 1982, p. 55ff; CIM Bulletin, November, 1978 p. 128ff, etc., and there are a number of corresponding patents. Also a substantially similar process is known as the Cominco process (Japanese Patent Publication No. 18057/81).
All of these processes substantially comprise blowing downward powdered lead sulfide concentrate together with oxygen or oxygen-enriched air into the combustion zone of a furnace so as to oxidize (burn) the concentrate in the gaseous phase. Thus crude lead and sulfur dioxide are produced. The oxidation of lead concentrate proceeds in accordance with the chemical equation: EQU PbS+O.sub.2 .fwdarw.Pb+SO.sub.2
This is an exothermic reaction and therefore is self-sustained once started.
Flash smelting is advantageous in that sintering, which has been a bottleneck in the conventional sintering-reduction process, is not required, fuel need not be used, and therefore the amount of the exhaust gas is small. However, it has the following defects.
1. The reaction of lead sulfide concentrate and oxygen occurs in the gaseous phase and the temperature of the flame (reaction system) reaches 1300.degree. C.-1700.degree. C., and PbO, which severely attacks the refractory, is produced. Thus the furnace wall is highly corroded.
2. The reaction proceeds in the gaseous phase at a higher temperature and volatile lead compounds such as PbO, PbS, etc. are volatilized so that much flue dust is formed. Therefore, the primary yield of metallic lead is very low, and energy consumption is high for treatment of the recycled flue dust. (In this respect the temperature at the top of the smelting furnace in the conventional sintering-reduction process is far lower, and volatilized valuable substances are automatically recovered.)
For the above-described reasons, the flash smelting of lead is not yet successfully employed in a commercial scale.
As a direct process of lead smelting in which the sintering step is eliminated, a process comprising blowing lead sulfide concentrate together with free-oxygen-containing gas into a molten lead in a converter through the tuyere (bottom blowing converter process) is disclosed in U.S. Pat. No. 3,281,237 (1966). In this process, the reactants are blown in through the tuyere, and therefore, there occur rapid exothermic reactions and vigorous agitation in the vicinity of the tuyere, which attack the refractory therearound. Therefore, more than 30% oxygen cannot be contained in the blowing gas, and thus this process cannot be successfully practised.
In Japanese Patent Publication No. 21059/81, Boliden's direct smelting of lead concentrate using a top blowing rotary converter is disclosed. This process comprises top blowing of oxygen-enriched air, but the blast pressure is low and agitation of the melt depends on rotation of the converter. Melting and oxidation of the concentrate and reduction of the slag are carried out in separate steps in the same furnace, and therefore, it is not a continuous process like the converter process.
In Canadian Pat. No. 893,624 (1972) and Japanese Laying-Open Patent Specification No. 47801/75, N.J. Themelis et al's continuous process of lead smelting (bottom blowing single furnace process) is disclosed. In this process, pelletized lead concentrate is introduced into a horizontal furnace at one end thereof, free-oxygen-containing gas is blown into the melt through a plurality of tuyeres provided at the bottom along the length of the furnace so as to oxidize the lead sulfide, and a reducing agent is supplied into the furnace near the other end of the furnace in order to reduce PbO in the slag. This process has the abovementioned defects of the bottom blowing process, and also has the defect of the single furnace process as described below.
Loss of lead into the slag is generally high in the direct smelting process. Therefore, it is necessary to recover the lead from the slag to return into the crude lead melt by reducing the slag. In the above-described flash smelting, this reduction is effected in the same furnace as the flash oxidation in the Kivcet process, and it is effected in a separate furnace in the Outokumpuprocess. Designate the former process the "single furnace process" and the latter the "separate furnace process". Then in the single furnace process, it is required that the PbO content in the slag be enhanced and the sulfur content in the crude lead melt be reduced by increasing oxygen activity in one part of a furnace, while the lead in the slag is reduced by decreasing the oxygen activity in the other part. It is very difficult to maintain separate zones remarkably different in the oxygen activity in one furnace. In order to realize such a furnace, the furnace must be of a large scale and complicated auxiliary equipment is required, and a large amount of energy is consumed therefor.
In the field of copper smelting, a continuous copper smelting process, which continuously carries out operations from feeding of copper ore to recovery of crude copper, was established by Mitsubishi Kinzoku Kabushiki Kaisha (Japanese Patent Publication No. 43015/76, U.S. Pat. Nos. 3,890,139 and 3,901,489). In this process, matte and crude copper are produced in three furnaces which are connected with launders. In the first furnace, a smelting furnace, matte and slag are formed by blowing in dried copper concentrate powder and flux powder (siliceous sand, lime, etc.) with oxygen-enriched air through a lance and rapidly melting the feedstock. The formed matte and slag are transferred to the second furnace, a settler, wherein the matte and slag are separated, the slag overflows out of the furnace and is discarded, while the matte is transferred to the third furnace, a reduction furnace. In the third furnace, the matte is reduced to crude copper by supplying a flux (limestone) and oxygen-enriched air through a lance. In a preferred embodiment of this invention, the flux and oxygen-enriched air are blown in in the vicinity of the melt surface.
Although applicability of this process to nickel smelting and cobalt smelting is mentioned in the specification of said patent, the possibility of application of said process to lead smelting was unknown.
We studied prior art processes in detail, and, in view of the fact that PbS considerably dissolves in molten lead, we found that the above-mentioned defect of the flash smelting can be overcome by employment of top blowing as in the above-mentioned copper smelting process; and that the reduction of slag can be more efficiently effected by employment of a separate furnace. Through further research, we have now completed the present invention.